President Barack Obama made a final pitch to Congress to act on the fiscal cliff on Sunday, citing Republican intransigence as Senate leaders struggled to bash out a last-minute deal.

In a rare foray onto the weekly round of political talk shows, the president sought to put the blame for looming economic crisis firmly at the door of the Republican party, accusing his opponents of having "trouble saying yes" to any proposal put before them.

"They say that their biggest priority is making sure that we deal with the deficit in a serious way, but the way they're behaving is that their only priority is making sure that tax breaks for the wealthiest Americans are protected. That seems to be their only overriding, unifying theme," Obama said.

The pointed remarks came as lawmakers in Washington prepared for a rare Sunday session. But with hours to go before they were due to sit, it was unclear if politicians would have any fiscal cliff legislation to debate.

An unnamed senior Senate aide told Reuters on Sunday afternoon, shortly before a 3pm target for an agreement, that negotiators had not yet reached a deal to present to their colleagues.

Congress has until midnight on Monday to find a solution to the current fiasco. That deadline will automatically trigger a series of fiscal measures that experts have said could plunge the US back into a recession.

If no deal is done, 88% of Americans will see their taxes rise on 1 January, a wave of deep spending cuts will start to take effect, and 2 million long-term unemployed people will lose their benefits.

The task of reaching a compromise has fallen on Harry Reid, the Democratic Senate majority leader, and Mitch McConnell, who heads the Senate's Republican minority.

Both men were summoned to the White House on Friday, alongside House speaker John Boehner and minority leader Nancy Pelosi. Treasury secretary Timothy Geithner was also in attendance.

After that meeting, Obama said he remained "modestly optimistic" that a deal could be achieved.

But since then there has been no firm indication that a grand compromise was indeed obtainable.

"I was modestly optimistic yesterday, but we don't yet see an agreement. And now the pressure's on Congress to produce," Obama told NBC's Meet the Press on Sunday.

Boehner responded by saying Americans elected the president to lead, not cast blame. "The president's comments today are ironic as a recurring theme of our negotiations was his unwillingness to agree to anything that would require him to stand up to his own party," Boehner said. "We've been reasonable and responsible. The president is the one who has never been able to get to 'yes'."

The main sticking point is the threshold for raising income taxes on households with upper-level earnings.

Obama wants all earners of $250,000 a year and above to shoulder a greater burden. Analysts believe that any deal could be anchored on raising taxes for households earning more than $400,000 or $500,000 a year.

But many Republicans in the House have indicated that they will vote against any increase in tax.

Not only could this scupper the chances of a grand deal, it could also see the blocking of the White House's back-up plan to avert the fiscal cliff.

Obama has indicated that if Reid and McConnell fail to produce an agreement by the end of Sunday, he will strong-arm Congress into a vote on scaled-back measures that would avert the immediate cost of America heading over the fiscal cliff.

That simple "up-or-down vote on a basic package" would stop tax hikes for middle-income Americans, while "laying the groundwork for future progress on more economic growth and deficit reduction", Obama said on Saturday.

But even that may have difficulty passing through the House, given the entrenched position of some Tea Party-backed Republicans.

Obama appeared to prepare for that eventuality on Sunday.

"If all else fails, if Republicans do in fact decide to block it, so that taxes on middle class families do in fact go up on 1 January, then we'll come back with a new Congress on 4 January and the first bill that will be introduced on the floor will be to cut taxes on middle class families," he said.

But he warned that missing the deadline would still result in "adverse reaction in the markets" and would "hurt our economy badly".